The quest to catalogue Earth's rich flora has taken botanists to the farthest flung and most treacherous corners of the world, from the humid rainforests of the Amazon to the highest peaks of Borneo.

Which made it all the more surprising when Iain Darbyshire stumbled upon a species of plant unknown to science while taking a lunchtime stroll around the Royal Botanic Gardens in west London.

Darbyshire, an expert in African botany at Kew, happened upon the foot-tall plant in full bloom, its striking green and grey heart-shaped leaves set off by tiny white and pink flowers.

"I just happened to take a different route through the glasshouse that lunchtime and stumbled across it," Darbyshire told the Guardian. "I knew instantly that it was a new species. It was just sat there waiting for someone to study it."

Record books revealed the plants had been donated by Swedish botanists in the 1990s after an expedition to the Eastern Arc mountains of Tanzania. Unsuspecting gardeners had tended them for more than a decade, using them as tropical bedding in Kew's Princess of Wales Conservatory.

The plant was officially named Isoglossa variegata last month and is among more than 250 new plant and fungus species discovered and described by the gardens' botanists in the past year.

Almost a third of all the species are believed to be facing extinction as their habitats are eroded or destroyed by logging, climate change and other environmental disruption.

In western Madagascar, Kew botanists hiked across extraordinary landscapes of limestone pinnacles and discovered several new species of wild coffee plant, the most traded commodity in the world after oil.

This unique environment has given rise to coffee plants that look nothing like those found elsewhere. Some of the species are conspicuously hairy, and two, Coffea labatii and Coffea pterocarpa, have colourful winged fruit.

The region experiences torrential seasonal downpours that create ephemeral rivers and pools across the stoney forest floor. "These winged fruit float very well, so the feature might be an evolutionary adpatation to aid their dispersal," said Aaron Davis, a coffee expert and taxonomist at the Gardens.

Alternatively, the wings may ensure the fruit are scattered far and wide by making them more visible to lemurs, which feed on the coffee beans.

The hirsute coffee plants might have sprouted hair to protect against harsh ultraviolet rays in the dry season.

"There's a misconception that we've found all the plants there are to find, but we are still in a golden age of discovery," said Davis. "We don't know our planet well enough and we are running out of time. Species are going extinct before we even know about them."

Around 70% of wild coffee species are in danger of extinction.

Elsewhere in Madagascar, botanists noticed two new species of small flowering plants called Gymnosiphon. The bizzare plants draw their energy not from the sun, but from fungi that live underground.

Further expeditions to the rainforests of Cameroon led to the discovery of three giant trees that grow to more than 30m high. One, Berlinia korupensis, is a member of the pea family. The tree towers above its neighbours at 42m high and produces foot-long pods that explode when they ripen, propelling seeds far across the forest floor.

Among some of the smallest species identified this year are tiny wood-rotting fungi from Australia that are less than a millimetre wide and cover trees like a thin coating of paint.

"They are small, but they perform a vital role in decomposition of plant material and recycling of nutrients," said Brian Spooner, a Swedish fungus expert working with Kew researchers.

In South Africa, botanists spotted a plant with lumpy wooden tubers that grow up to a metre high. The species was identified as a yam, but only 200 or so are known to exist in the wild. It is under threat from local medicinal plant collectors who use it as a treatment for cancer.

Some 20 new species were discovered in Brazil alone, the most striking being a red passion flower that is probably pollinated by hummingbirds and produces edible egg-shaped fruit. The plant was spotted in an expedition to the Amazon rainforest in Mato Grasso, Brazil.

The largest haul of new species came from Mount Kinabalu, the highest peak in Borneo, where botanists Jeff Wood and Phil Cribb have identified 38 new species of orchid. Nearly 900 different species live in a 1,200sq km area of the island.

Each new species is identified by detailed visual inspections that are often backed up my genetic analyses. To identify all the world's flora could take another 50 years, but the effort is crucial for conserving rare species and reintroducing species that only exist in protected areas.

Stephen Hopper, director of the Royal Botanic Gardens Kew, said the speed of discovery and classification of new species is increasing under the organisation's Breathing Planet Programme.

"These new discoveries highlight the fact that there is so much of the plant world yet to be discovered and documented. Without knowing what's out there and where it occurs, we have no scientific basis for effective conservation," said Hopper.